Wide-band matching network for piezoelectric transducer

ABSTRACT

Wide-band network for matching a signal-conducting line having a characteristic impedance of 50 ohms to a piezoelectric transmitter having a capacitance C o  and a reactance X o  for a center frequency f o  of the frequency band to be transmitted. Various circuit arrangements and dimensioning rules for the circuit elements are specified as a function of the reactance X o . The transmitter is a part of an acoustic lens arrangement.

BACKGROUND OF THE INVENTION

The present invention relates to a wide-band network for matching asignal-conducting line having a characteristic impedance of 50 ohms to apiezoelectric transmitter having a capacitance C_(o) and a reactanceX_(o) for a center frequency f_(o) of the frequency band to betransmitted.

Piezoelectric transmitters of this type are used especially inacoustical microscopy for converting high-frequency oscillations intoultrasonic waves and back again. They are mounted in direct acousticcontact on a second propagation medium, for example sapphire. Normally,they consist of a sputtered zinc oxide layer (ZnO) which is embeddedbetween two metal layers, one of which is sputtered onto the sapphire.If then an electric field of a suitable frequency is built up betweenthe metal layers, the ZnO layer generates an acoustic field. Thethickness of the layer must be selected in accordance with the workingfrequency desired for the acoustical microscopy. Typical workingfrequencies are between 100 MHz and 2 GHz. The thickness of the ZnOlayer is selected to be between λ/4 and λ/2 of the wavelength of theworking frequency within the ZnO layer.

As far as the electric connection is concerned, such a transmitteressentially represents a capacitive impedance. Its equivalent circuitcan be represented as a resistance R.sub.Ω in series with a capacitanceChd o. Typical values for the resistance are about 1/10 of the reactanceof the capacitor at the center of the frequency band to be transmittedif the Q of the transmitter is 10, the quality factor Q being defined ascapacitor reactance divided by the resistance.

Direct coupling of the transmitter to a 50-ohm system would bring withit large power losses because of the large electric mismatch. For thisreason, electric matching networks are used between the transmitter andthe 50-ohm system which are usually optimized for a certain workingfrequency so that the matching applies only to a small range around thisfrequency. Therefore, the matching networks have only a limitedbandwidth.

For applications covering a large range of bandwidths, it is desired tooptimize the matching network over a larger range. This is generallysuccessful only to the extent to which the number of matching elementsis also increased. A theoretical upper limit for expanding the bandwidthis essentially determined by the Q of the transmitter.

The practical possibility of implementing the network is made more andmore difficult as the number of components increases. For this reason,the effort is made for practical reasons to limit the number of matchingelements.

For application in the field of acoustical microscopy, it must also benoted that the matching network is necessarily a part of the acousticalless arrangement. For image scanning, this lens must be moved in anoscillatory fashion. Thus, a need exists for miniaturizing the matchingnetwork so that the masses to be moved can be kept as small as possibleso as to expend as little energy as possible.

SUMMARY OF THE INVENTION

The present invention is therefore directed toward providingconfigurations for the matching network which achieve a maximum ofbandwidth with a minimum number of components. In particular, abandwidth of f_(o) /2 should be possible for a ZnO transmitter.

According to the invention, this object is achieved through theprovision of matching networks for piezoelectric transmitters of thetype initially mentioned, accomplished by selecting the circuit as afunction of the reactance of the transmitter. Advantageous dimensionvalues for the components of the matching networks are selected in asimilar manner. A circuit containing components having these selectedvalues can be implemented in particularly advantageous manner atfrequencies f_(o) which are in the gigahertz range in accordance with afurther aspect of the invention.

Specifically, the present invention encompasses three circuits usedalternately depending upon the reactance X_(o) of the piezoelectrictransmitter. Where this reactance is not greater than 50 ohms, thesignal-conducting line is connected to the electrode of the transmitterthrough a series combination of a capacitor and an inductor. Moreover,the electrode is also connected through a second inductor to ground.When the reactance is between 50 and 250 ohms, the signal-conductingline is connected to the electrode through an inductor. The signalconducting line is also connected to ground through a parallelcombination of a capacitor and a second inductor. Finally, when thereactance is greater than 250 ohms, the signal conducting line isconnected to the electrode through a series combination of an impedanceand an inductor. Approximate values for the capacitors, inductors, andimpedance are defined in accordance with the capacitance and reactanceof the piezoelectric transmitter and the center frequency of the signalbeing transmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bemore clearly understood from the following written description read inconjunction with the drawings, in which:

FIG. 1 shows an acoustic lens arrangement known per se;

FIG. 2 shows the equivalent circuit for obtaining a match to atransmitter having a reactance of X_(o) less than 50 ohms;

FIG. 2a shows an alternate embodiment of the invention of FIG. 2;

FIG. 3 shows the equivalent circuit at a reactance of X_(o) which isbetween 50 and 250 ohms inclusive; and

FIG. 3a shows an alternate embodiment of the invention of FIG. 3;

FIG. 4 shows the equivalent circuit at a reactance of X_(o) greater than250 ohms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an acoustic lens arrangement as used in acousticalreflection microscopy. On a plane surface of a sapphire rod 1 apiezoelectric transmitter is mounted which consists of a first goldlayer 2, a ZnO layer 3 sputtered thereon and of a second gold layer 4.The two gold layers 2 and 4 are connected to the matching network 5 inwhich arrangement the gold layer 4 is the active electrode of thetransmitter. The gold layer 2 is connected to ground potential.

The matching network 5 is preceded by a circulator 6 which, during afirst period, passes the signal 7 coming from a generator to thetransmitter and, in a second period, blocks this signal and passes themeasurement signal coming from the transmitter to the output lines 8.The signal-conducting line is designated by 9.

The capacitance C_(o) of the transmitter can be determined on the basisof the known area A, the thickness d and the dielectric constant ε ofthe piezoelectric layer 3:

    C.sub.o =(ε·A)/d

For an intended center frequency f_(o) of the frequency band to betransmitted, the reactance X_(o) is then found as: ##EQU1##

The matching network shown in FIG. 2 is particularly suitable for atransmitter reactance X_(o) less than 50 ohms. In this arrangement, thecircuit elements L₁, C₁ and L₂ have approximately the followng values:##EQU2##

At frequencies in the GHz range, this network can be implemented byconnecting two connecting wires to the active electrode 4 of thetransmitter. One of these wires is connected to ground to form theinductance L₁ and the other one is connected to a chip capacitor C₁ toform the inductance L₂. At a center frequency f_(o) =1 GHz, thismatching network can be used for a frequency range from about 750 MHz to1,250 MHz. This is a range which is preferred for acoustical microscopy.If transmitters having a reactance X_(o) of less than 50 ohms are used,this makes it possible to construct acoustic lens arrangements, thegeometric dimensions of which are only determined by mechanical holderparts and electric connecting elements.

An alternate embodiment of the invention is shown in FIG. 2a wherein theinductors L₁ and L₂ comprise wires.

The matching network shown in FIG. 3 is particularly advantageous for areactance range of from 50 to 250 ohms. In this arrangement, the circuitelements L₃, C₂ and L₄ have approximately the following values: ##EQU3##Here too, for frequencies in the GHz range, a possibility forimplementation is created which simplifies considerably the geometricconfiguration of the matching network. This is because L₃ can befabricated by appropriately choosing the length of the connecting wireto the active electrode 4 of the transmitter in a manner which will beapparent to one having ordinary skill in the art. Such an arrangement ofa wire inductor is shown in FIG. 3a.

The matching network, shown in FIG. 4, for transmitter reactances ofX_(o) greater than 250 ohms uses circuit elements having approximatelythe following values: ##EQU4##

The geometric dimensions of this network are essentially determined bythe necessary impedance Z which is usually formed by a strip waveguidethe length of which is a quarter of the wavelength λ at the centerfrequency f_(o).

What is claimed is:
 1. A wide band network comprising means for matchingthe impedance of a signal-conducting line having a characteristicimpedance of about 50 ohms to an electrode, a piezoelectric transmitterhaving said electrode as a part thereof, said transmitter having anequivalent ohmic resistance R.sub.Ω series with an equivalentcapacitance C_(o), the resistance R.sub.Ω being approximately one-tenthof the reactance of said capacitance C_(o), and a reactance X_(o) notgreater than about 50 ohms at a center frequency f_(o) of the frequencyband of signals to be transmitted and said matching means comprising:aseries combination of a first capacitor having capacitance C₁ and asecond inductor having an inductance L₂ connecting saidsignal-conducting line to said electrode; and a first inductor having aninductance L₁ connecting said electrode to ground; wherein the valuesC₁, L₁, and L₂ are determined approximately by the followingrelationships: ##EQU5##
 2. A wide band network as claimed in claim 1,wherein, for f_(o) greater than about 1 GHz, said first and secondinductors respectively comprise wires attached to said electrode, andsaid first capacitor comprises a chip capacitor.
 3. A wide band networkfor matching the impedance of a signal-conducting line having acharacteristic impedance of about 50 ohms to an electrode of apiezoelectric transmitter, said transmitter having an equivalent ohmicresistance R.sub.Ω in series with an equivalent capacitance C_(o), theresistance R.sub.Ω being approximately one-tenth of the reactance ofsaid capacitance C_(o), and a reactance X_(o) between about 50 to 250ohms at a center frequency f_(o) of the frequency band of signals to betransmitted, comprising:a first inductor having an inductance L₃connecting said signal-conducting line to said electrode; a firstcapacitor having a capacitance C₂ connecting said signal-conducting lineto ground; and a second inductor having an inductance L₄ connected inparallel with said first capacitor and also connecting saidsignal-conducting line to ground; wherein the values C₂, L₃, and L₄ aredetermined approximately by the following relationships: ##EQU6##
 4. Awide band network as claimed in claim 3 wherein, for f_(o) greater thanabout 1 GHz, said first inductor comprises a wire.
 5. A wide bandnetwork comprising means for matching the impedance of asignal-conducting line having a characteristic impedance of about 50ohms to an electrode, a piezoelectric transmitter having said electrodeas a part thereof, said transmitter having an equivalent ohmicresistance R.sub.Ω in series with an equivalent capacitance C_(o), theresistance R.sub.Ω being approximately one-tenth of the reactance ofsaid capacitance C_(o), and a reactance X_(o) not less than about 250ohms at a center frequency f_(o) of the frequency band of signalstransmitted, and said matching means comprising a series combination ofa λ/4 length impedance line having a characteristic impedance Z and aninductor having an inductance L₅ for connecting said signal-conductingline to said electrode, where λ is the signal wavelength at centerfrequency f_(o) ;wherein the values Z and L₅ are determinedapproximately by the following relationships: ##EQU7##